Currency, and other negotiable paper which are in circulation eventually becomes so worn as to be unfit for continued use. The U.S. Treasury Department daily removes from circulation millions of dollars which have deteriorated in quality below acceptable standards. Until recently all of this inspection was done by human inspectors who visually examine each bill and make a determination of whether a bill is fit or unfit for further circulation. While these individuals are highly skilled, such a routine function is slow, inefficient, wasteful of human resources and subject to non-uniformity in the judgment standards among different individuals.
Various techniques have been devised for processing automatically both uncirculated and circulated currency. Systems for processing uncirculated currency are primarily for the purpose of detecting flawed bills which are then prevented from entering circulation.
Systems for processing circulated currency primarily check for such things as quality, genuineness, and denomination. Both systems may employ automatic sorting arrangements.
The amount of dirt on a bill is an excellent measure of its quality since general wear, wrinkles, limpness, etc. are all closely related to the amount of dirt that has accumulated on the bill during its circulation. Thus, quality of a bill may be determined by measuring the dirt it has picked up.
One method of making such a measurement is by detecting and converting to a signal the amount of light transmitted through the bill and comparing the signal to a reference signal representative of a bill of acceptable quality. One such system is described broadly in U.S. Pat. No. 3,976,198 entitled "Method and Apparatus for Supporting Currency". The method therein disclosed measures transmissivity of light through the bill over its length during transit through an inspection station. The total amount of light transmitted through the bill is averaged or integrated to produce a signal representative of total transmissivity of the bill. This signal is compared to a known standard for determination of fitness or unfitness of the bill. Systems such as the above described patented system which rely on transmissivity alone for a quality check are prone to errors caused by circuit variations and variations in light intensity, such as, transient changes in intensity due to light source voltage variations as well as degradation of intensity in light emission over the lifetime of the light source.
The present invention relates to an apparatus for measuring quality of currency which compensates for errors due to light source and circuit variations.